The invention relates to an energy converter for supplying electric energy from an energy source to a load, the energy converter comprising a transformer having a primary side and a secondary side, the secondary side being adapted to be connected, in operation, to the load, at least a first and a second, series-arranged, controllable switch to be connected, in operation, to the energy source for generating an alternating current in the primary side of the transformer, diodes arranged anti-parallel to the first and the second switch, and a control device for generating control signals with which the first and the second switch are opened and closed, the control device comprising detection means for generating a detection signal when the energy converter is operative in a capacitive or near-capacitive mode.
An energy converter of this type is known per se from, inter alia, U.S. Pat. No. 5,075,599 and U.S. Pat. No. 5,696,431. In this converter, the load is often a rectifier and the energy source is a DC voltage source. Together with the load, the energy converter has for its object to convert a DC input voltage of the energy source into a DC output voltage of the load. However, the load may also comprise a different device than the rectifier, which device is fed with an alternating voltage. The energy converter may thus consist, inter alia, of a DC/DC converter and a DC/AC converter.
For a satisfactory operation of the energy converter, it is important that the switches for generating the alternating current are switched on and off at the right instant. The frequency at which the switches are switched on and off defines the mode of operation of the converter. If the frequency is sufficiently high, the energy converter operates in a regular inductive mode. In this mode, the phase of the current through the primary side of the transformer trails the phase of the voltage at the node. After a current-conducting switch is opened, and after the diode of the other switch has started to conduct the current, the other switch can be opened. In that case, there are no switching losses. The time interval in which both switches are opened is referred to as the non-overlap time.
The converter operates in the near-capacitive mode when the switching frequency of the switches, and hence the frequency of the alternating current through the primary side of the transformer is decreased to a point where the alternating current is at least almost in phase with the alternating current at the node. After the current conducting switch is opened and before the diode, which is arranged anti-parallel to the other switch, starts to conduct, the direction of the current through the primary side of the transformer is reversed. Hard-switching takes place if the other switch is closed in that case. This means that switching takes place at an instant when there is a voltage difference across the relevant switch. This will result in switching losses.
The converter operates in the capacitive mode when the frequency at which the switches are switched is further decreased to a point where the alternating current through the primary side of the transformer is in phase with, or even leads the phase of the voltage at the node. The switching losses also occur in this mode.
Generally, it is desirable that the energy converter operates in the inductive mode.
The known detection means are often used to prevent the energy converter from operating in the near-capacitive mode or in the capacitive mode. If the near-capacitive mode is detected, the control device may raise the frequency at which the switches are switched so that the converter will certainly start working again in the inductive mode. The frequency may be raised in a number of small steps per cycle of the converter, or in one big step, all this being dependent on detection of either the near-capacitive mode or the capacitive mode.
In accordance with the state of the art, two methods of detecting the (near-) capacitive mode are known. First, it is known that the detection means determine whether the converter operates in the near-capacitive mode with reference to the current through the converter during the non-overlap time, or with reference to the polarity of the current of the converter. This method is known from U.S. Pat. No. 5,075,599. In the near-capacitive mode, this current is small with respect to this current in the inductive mode. In the capacitive mode, the polarity of the current is opposed to the polarity of the current in the inductive mode. The amplitude of the current is therefore often compared during the non-overlap time with the reference value for determining whether the energy converter is operative in the (near-)capacitive mode.
Secondly, it is known to detect a current peak across a capacitor which is incorporated between the node and, for example, one of the terminals of the energy source. This method is known from U.S. Pat. No. 5,696,431. If such a current peak occurs, it is an indication that the energy converter switches hard and is therefore operative in the (near-)capacitive mode.
The known techniques provide the possibility of detecting whether the converter is operative in the capacitive mode or in the near-capacitive mode. One of the most important reasons for ensuring that the energy converter is not operative in the (near-) capacitive mode is the dissipation which occurs in the switches due to hard-switching. Hard-switching may indeed be minimized by means of the known techniques described above. It can therefore be prevented by means of the known techniques that hard-switching takes place because, in the case of detection of the capacitive or near-capacitive mode, the frequency of the energy converter is adapted in such a way that the converter becomes operative in the inductive mode again.
A drawback of the known method in which the current through the converter, or the polarity of the current through the converter is determined during the non-overlap time is that the control device adapts the frequency in such a way that the energy converter becomes amply operative in the inductive mode when the detection means of this control device detect that the energy converter is operative in the capacitive mode or the near-capacitive mode. Amply operative in this respect means that the frequency is raised more than is necessary to cause the converter to operate in the inductive mode. This in turn means that the range of the power which can be supplied to the load is unnecessarily limited.
The method in which a current peak is detected is only suitable for detecting hard-switching as such. It is not possible for determining the amplitude of hard-switching. In fact, hard-switching takes place when a switch is closed at the instant when there is still no voltage difference across the switch. This voltage difference is a measure of hard-switching. The larger the voltage difference, the harder switching takes place and the larger the switching losses in the switches. For this reason, the latter method is only suitable for adapting the frequency in such a way that the converter becomes operative in the inductive mode again when hard-switching has been detected. There is no question of a fine control with which the converter can be just brought to the inductive mode without raising the frequency to an unnecessarily high extent.
It is an object of the invention to provide an energy converter with which the drawbacks described can be alleviated, if desired. The invention is also based on the recognition that it will provide a great advantage when it is possible to define the amplitude upon hard-switching. The foregoing means that it is desirable to determine the voltage across a switch just before the instant when it is closed. In that case, it has been made possible to create a control loop, if desired on the basis of this information, which control loop utilizes said voltage difference across the relevant switch in a feedback circuit for controlling the frequency at which the switches of the energy converter are switched. In other words, a control loop can be created for controlling the frequency of the alternating current generated by the energy converter in the transformer. The frequency of the energy converter can thus be controlled in such a way that there is only a small voltage difference across the switch at the instant when it is switched, so that, in the inductive mode, switching takes place near the boundary of the near-capacitive mode. It is thereby achieved that the output power of the converter has a maximal range. Accordingly, the invention is characterized in that, for the purpose of generating the detection signal, the detection means are adapted to detect a voltage jump which occurs at a node between the first and the second switch when the first or the second switch is closed.
Since, according to the invention, the voltage jump is measured, it can be determined very accurately in how far the energy converter is operative in the capacitive mode or the (near-)capacitive mode. Since the mode in which the energy converter is operative is accurately known, the frequency of the energy converter can be adapted very accurately accordingly and as desired.
Particularly, it holds that the value of the detection signal is a measure of the value of the voltage jump.
In accordance with a further elaboration of the invention, it holds that the detection means for generating the detection signal are adapted to detect a voltage jump which occurs at a node between the first and the second switch when the first or the second switch is closed. The detection signal may then be formed by the voltage Vdiv or a related quantity. Particularly, it holds that the switching frequency at which the first and the second switch are switched is adjusted by the control device in dependence upon the detection signal. This adjustment may be such that the frequency is operative in the inductive mode, however, bordering on the near-capacitive mode. In that case, the power that can be supplied by the energy converter has a maximal range. To this end, particularly the control device is adapted to adjust, in operation, the switching frequency in such a way that the value of Vdiv reaches a selected relatively small value.
In operation, the control device will re-open the short-circuit switch after the sample-and-hold circuit has determined the voltage Vdiv. The sample-and-hold circuit preferably retains the voltage Vdiv until the new value of Vdiv is determined. The detection signal is therefore preferably equal to the most current value of Vdiv.
These and other aspects are apparent from and will be elucidated with reference to the embodiments described hereinafter.